1. Field of the Invention
The present invention relates to a switching apparatus including a plurality of semiconductor switching elements, for example, transistors.
2. Description of the Related Art
FIG. 7 is a circuit diagram showing a conventional switching apparatus. In the figure, reference numeral 1a and 1b each denote an insulated gate bipolar transistor, which is a semiconductor switching element, (hereinafter referred to as an IGBT). Each IGBT has a pair of main circuit electrodes, that is a collector C and an emitter E. Each IGBT is further provided with a terminal (e) at of the emitter E which is at a reference potential, and has a control electrode (g) which is insulated from the collector C and the emitter E. Reference numeral 2 denotes a bus line of a power supply; reference numeral 3 denotes a bus line connected to a load; reference numeral 4 denotes a first connection conductor from bus line 3 to IGBT 1a; reference numeral 5 denotes the inductance of the connection conductor 4; reference numeral 6 denotes a first connection conductor from bus line 3 to IGBT 1b; and reference numeral 7 denotes the inductance of the connection conductor 6. Of two IGBTs 1a and 1b, their collectors C are connected to the power-supply bus line 2 respectively via second connection conductors 4a and 6a; their emitters E are connected to the bus line 3 through the first connection conductors 4 and 6, respectively.
Reference numeral 10 denotes a control signal source which is connected to the two IGBTs 1a and 1b. The control signal source 10 has two voltage sources incorporated therein: a first signal source 11 and a second signal source 12 connected as shown in FIG. 7. The negative side of the first signal source 11 and the positive side of the second signal source 12 are connected to a terminal 13. The positive side of the second signal source 11 and the negative side of the second signal source 12 are connected to a terminal 14 via an unillustrated switching circuit.
Reference numerals 15 and 18 each denote a first connection line, and reference numerals 17 and 20 each denote a second connection line. The connection lines each connect the terminals 13 and 14 of the control signal source 10 with the terminals (e) and control electrodes (g) of the IGBTs 1 on the right and left sides, as shown in FIG. 7. Reference numeral 16 schematically denotes the inductance of the connection line 15; reference numeral 19 schematically denotes the inductance of the connection line 18.
Next, the operation of the conventional switching apparatus will be explained. The voltage of the first signal source 11 is applied by closing an unillustrated switching circuit, so that the terminals (e) become negative and the control electrodes (g) become positive. As a result, the sections between the collectors C and the emitters E conduct in response to the applied voltage, thereby allowing a load current to flow from the power-supply bus line 2 to the load-side bus line 3. If a predetermined voltage of the second signal source 12 is applied so that the terminals (e) become positive and the control electrodes (g) become negative, the conductance between the collector C and the emitter E is blocked, thereby shutting off current flowing from the power-supply bus line 2 to the load bus line 3.
Semiconductor switching elements generally have variations in the time (turn-on/turn-off time) from the application of a switching signal to their control electrodes until the section between main circuit electrodes is open or closed. Also, connection conductors and connection lines for connecting semiconductor switching elements or the like have inductance.
It will now be assumed that the IGBTs 1a and 1b are opened by a voltage signal from the control signal source 10, whereby a current flowing from the collector C of each of IGBTs 1a and 1b to the emitter E thereof is shut off. If the turn-off time of the IGBT 1a should be shorter than the turn-off time of the IGBT 1b, the current flowing through the IGBT 1a decreases earlier. Therefore, a voltage proportional to the product of the ratio of change in current which depends on the characteristics of the IGBT 1a and the inductance 5 of the connection conductor 4 on the left side develops across both ends of the inductance 5, with the voltage having the polarities shown in FIG. 7, i.e., the bus line 3 becomes positive and the emitter E of the IGBT 1a becomes negative. This developed voltage is divided into a plurality of voltages having polarities indicated by symbols .sym. and .crclbar. in FIG. 7 in proportion to the respective inductances on a series circuit formed by inductance 7, inductance 19 and inductance 16. As a result, a voltage smaller than or greater than the voltage applied from the control signal source 10 is applied to the control electrode (g) of the IGBTs 1a or 1b.
Since the conventional switching apparatus is constructed as described above, a voltage induced by inductances 5 and 7 at the time of control, e.g., at opening/closing time, is applied to inductances 16 and 19 between the control signal source 10 and the terminals (e) of the IGBTs, with the result that a voltage higher than or lower than the voltage supplied from the control signal source 10 is applied to the control electrode (g) of the IGBT. Hence, in some cases the IGBTs malfunction or in some cases IGBTs are destroyed because a voltage above an allowable voltage for IGBTs was applied thereto.